Many different types of photovoltaic devices are known in the art (e.g., see U.S. Patent Document Nos. 2004/0261841, 2006/0180200, U.S. Pat. Nos. 4,335,266, 4,611,091, 6,784,361, 6,288,325, 6,631,603, and 6,123,824, the disclosures of which are incorporated by reference herein in their entireties). Examples of known photovoltaic devices include CIGS (approximately Cu(In, Ga)(Se,S)2 and/or CuInX-1GaxSe2) solar cells. CIGS films are conductive semiconductor compounds that are often referred to as an absorber or light absorbing layer(s) or film. Generally speaking, CIGS type photovoltaic devices include, from the front or light incident side moving rearwardly, a front cover of material such as glass (front substrate), a front electrode comprising a transparent conductive layer(s) (e.g., a transparent conductive oxide such as zinc oxide), a light absorption semiconductor film (e.g., CIGS), a rear electrode/contact, and a rear substrate of a material such as, for example, glass (or metal foil for flexible applications). In some instances, an adhesive may be provided between the front cover glass and the front electrode. It is also the case in some instances that the device is provided with window layer(s) (e.g., of or including CdS, ZnS, or the like). Photovoltaic power is generated when light incident on the front side (or front cover glass) of the device passes through the front electrode and is absorbed by the light absorption semiconductor film (e.g., CIGS), as is known in the art. Certain designs may also utilize compositional grading of the semiconductor absorber, for example, with an increased Ga/(Ga+In) ratio toward the rear electrode or contact. Photovoltaic devices having a compositionally graded CIGS absorber may, for example, be made using a two- or three-step deposition process.
For example, with reference to FIG. 1, there is generally provided a schematic cross-sectional diagram illustrating various elements of a conventional CIGS-type photovoltaic device 10. The cell 10 is structurally supported on a rear glass substrate (or back glass) 12. A back contact comprising a metal layer, such as, for example, molybdenum (Mo) 14 is typically deposited on the glass substrate 12. The first active region of the device 10 comprises a semiconductor film 16 which is typically a p-type copper indium/gallium diselenide (CIGS). A thin “window” layer of n-type compound semiconductor 18, typically comprising cadmium sulfide (CdS) may then be formed on CIGS layer 16. A layer of conducting wide bandgap semiconductor material, typically formed of a substantially transparent conductive metal oxide, such as zinc oxide, is deposited on the CdS layer 18 and acts as a transparent front electrode 25 for the device 10. The device 10 may be completed by including a series of front face contacts (not shown) in the form of, for example, a metal grid on top of the transparent front electrode 25 to facilitate the extraction of generated electrons, and a front glass substrate 21. A large solar cell may also be divided into a number or smaller cells by means of scribes, such as, for example, laser or mechanical scribes or the like, traditionally referred to as P1, P2 and P3, which allow individual cells to be connected in series.
As noted above, a metal such as Mo may be used as the rear electrode (or back contact) 14 of a photovoltaic device, such as, for example, a CIGS solar cell 10, to extract positive charges generated in the CIGS semiconductor absorber 16 of the solar cell 10. In certain instances, the Mo rear electrode 14 may be sputter-deposited using, for example, direct-current magnetron sputtering, onto the back glass substrate 12 of the CIGS solar cell 10. There are certain advantages associated with using Mo as the material for the rear electrode (of course, it will be understood that Mo used in back contact configurations may include certain amounts of other elements and/or dopants of materials. Mo is inert to the CIGS absorber, and thus does not harmfully interfere with the CIGS and does not substantially alter the electrical, optical or mechanical properties of the CIGS. Using Mo to form the rear contact 14 may result in the formation of a thin molybdenum selenide (MoSe2) layer (not shown in FIG. 1) at the interface between the Mo rear electrode 14 and the CIGS film 16. The formation of a MoSe2 layer during the selenization process at the interface between the CIGS 16 and the Mo rear contact 14 provides an ohmic (e.g., non-rectifying) contact to the CIGS absorber which, in turn, facilitates hole extraction with reduced losses. Using sputter deposited Mo as the rear contact is also advantageous in that it is known to produce a surface morphology that is beneficial in CIGS growth for the formation of crystallites with large grain sizes that result in high carrier mobility, and thus higher efficiency photovoltaic devices.
It has been found that certain CIGS designs may benefit from reflecting light radiation (e.g., solar light) that initially passes through the CIGS absorber (16) without contributing to charge generation, back through the CIGS absorber, effectively recycling previously unused lost light by utilizing otherwise wasted light energy for charge generation.
For example, in arrangements that utilize compositional grading of the CIGS semiconductor absorber, for example, with increased Ga/(Ga+In) ratio toward the rear electrode or contact, the increased Ga concentration at the back of the photovoltaic device may lead to an increased bandgap of the absorber, which may result in lower absorption of incident light radiation (e.g., solar light), particularly in the near infrared region. This increased bandgap and corresponding lower absorption of light radiation reduces overall efficiency of the photovoltaic device because a portion of the light radiation (e.g., solar light) passes through the absorber without contributing to charge generation. The loss associated with incomplete light absorption typically increases with reduced thickness of the CIGS absorber.
According to certain example embodiments disclosed herein, it has been found that the MoSe2 interface layer formed between the Mo rear contact and the CIGS absorber during a high-temperature selenization process used during CIGS formation plays an important role in optical coupling of the reflected light. Moreover, according to certain embodiments disclosed herein, it has been found that at certain thicknesses, MoSe2 can provide improved optical coupling of the reflected light and significantly enhance the light absorbance of reflected light in the CIGS absorber.
According to example embodiments disclosed herein, it has been found that MoSe2 thicknesses that substantially correspond to maxima ranges of the absorbance of reflected light have little or substantially no dependence on the extinction coefficient and/or thickness of the CIGS absorber. It has also been found that these newly identified maxima ranges do not correspond to thicknesses of MoSe2 that are conventionally formed (unintentionally) at an interface of a Mo rear contact and CIGS absorber in prior art devices during selenization. Thus, it has been discovered that optimizing thickness of the MoSe2 layer can be achieved substantially independently of CIGS thickness and extinction coefficient. According to certain example embodiments disclosed herein controlling the MoSe2 thickness to desired range(s) provides increases in solar light absorption in the CIGS in the range of, for example, and without limitation, by at least about 5%, more preferably by at least about 10%.
In certain example embodiments disclosed herein, methods for making a photovoltaic device and/or a coated article for use in a photovoltaic device in which a thickness of the resulting MoSe2 layer is controlled to desirable range(s) is also provided. It is noted that intermediate coated articles may be provided by a large area thin-film coating facility/manufacturer to photovoltaic device manufacturers and may include foundational (or seed) layers of different material composition than the finally desired MoSe2 layer. These example foundational or seed layer(s) are subsequently used to form the MoSe2 having the desired thickness during high temperature selenization process used to form the CIGS that may be performed by a downstream photovoltaic device manufacturer.
In certain example embodiments disclosed herein, a back contact for a photovoltaic device includes a rear substrate (e.g., soda-lime-silica based glass or metal foil in certain flexible applications); a rear contact comprising a first conductive layer comprising or consisting essentially of molybdenum; a second conductive layer comprising or consisting essentially of MoSe2 having a desired thickness; a semiconductor absorber layer disposed above the MoSe2 inclusive layer; a front transparent electrode formed above the semiconductor absorber layer; and a front substrate (e.g. front substrate comprising glass). According to certain example embodiments, the preferred thickness of the MoSe2 inclusive layer may, for example, preferably be in range(s) corresponding to a maxima of the absorbance of reflected light in the CIGS absorber, such as, for example, and without limitation, in ranges including from about: 5-15 nm, 35-45 nm, 60-70 nm, and/or 90-100 nm, and more preferably at least one of about (“about” as used herein regarding these thicknesses means plus/minus 1 nm) 10, nm, 40 nm, 65 nm and/or 95 nm.
According to further example embodiments disclosed herein, a back contact structure for use in a photovoltaic device is provided, the back contact structure comprising: a rear substrate; and a back contact layer comprising a first Mo region having a first density and a second Mo region having a second density, said first density being more dense than said second density, said second Mo region being formed at an outer portion of the back contact layer above the first Mo region, said second Mo region forming a foundational seed region, wherein a thickness of said foundational seed region comprising Mo having the second density has a thickness in a range selected from the group consisting of: about 5-15 nm, about 35-45 nm, about 60-70 nm, and about 90-100 nm, and wherein the high density region of the back contact is located between the rear substrate and the foundational seed layer.
In order to achieve the example preferred thickness of the layer of or including MoSe2 discussed above, further example embodiments directed to methods for making a back contact for use in photovoltaic devices are disclosed. According to a first example embodiment, a foundational (or seed) layer of or including MoOx having a thickness substantially corresponding to a desired thickness of MoSe2 may be deposited, for example, (e.g., as a last step) after deposition of the Mo inclusive rear contact. During subsequent high-temperature selenization, such as, for example, a process used to form the CIGS absorber, selenium substantially replaces at least some oxygen in the foundational (or seed) MoOx layer, thus forming a MoSe2 inclusive layer having a desired thickness substantially corresponding to the thickness of the foundational MoOx based seed layer, and thus provide improved device performance, e.g., at or near a maxima of the absorption characteristics of CIGS and MoSe2.
In certain example embodiments, methods for making a back contact for use in a photovoltaic device are provided, comprising: providing a rear substrate; depositing a first conductive contact layer comprising Mo on (directly or indirectly) and/or over (“over” also covers both directly and indirectly on) said rear substrate; and forming (e.g., depositing) a foundational seed layer comprising or consisting essentially of MoOx above/over said first conductive contact layer comprising or consisting essentially of Mo, said foundational seed layer having a thickness selected from the group consisting essentially of: from about 5-15 nm, from about 35-45 nm, from about 60-70 nm, and from about 90-100 nm. According to certain example embodiments, the method may further include heat treatment in a selenium inclusive atmosphere to form a layer of or including MoSe2 out of at least the foundational seed layer.
According to another example embodiment, an outermost (or final) portion of the Mo rear contact may deposited and/or formed in such a manner that it is more susceptible to selenization during the high-temperature selenization process used in forming the CIGS absorber. For example, a lower density Mo layer/portion of the rear contact is more susceptible to selenization during the high-temperature selenization process used to form the CIGS absorber than is a higher density Mo used to form a bulk of the rear contact. According to an example embodiment, the deposition conditions for depositing an outermost portion of Mo based film (a “film” can include one or more layers), having a thickness substantially corresponding to a desired thickness of a future to-be-formed MoSe2 inclusive layer, may be altered such that a desired thickness of Mo having a lower density is formed as a foundational (or seed) layer or portion above a higher density Mo that forms a bulk of the rear electrode. For example, and without limitation, the lower density Mo foundational (or seed) portion or layer may have a density that is preferably at least about 5% lower, and more preferably at least about 10% lower, and more preferably at least about 20% lower, than that of the underlying higher density bulk of the Mo rear contact. An example deposition condition that results in lower density Mo deposition may include sputter depositing Mo at a higher pressure and lower power than the underlying higher density bulk of the Mo rear contact. For example, and without limitation, power-to-pressure ratio of the sputtering process used to form the more dense Mo portion of the rear contact may preferably be in a range of about 1/3 to about 1/20, and more preferably about 1/10, while the power-to-pressure ratio of the sputtering process used to form the less dense Mo portion of the Mo rear contact may preferably be in a range of 1/21 to 1/30, and more preferably about 1/25. An example sputtering process according to certain example embodiments may be direct-current magnetron sputtering. The lower density Mo portion of the rear contact may optionally include oxygen or other material(s), and regardless of whether it includes oxygen or not it is more susceptible to selenization during the high-temperature selenization process used to form the CIGS absorber, and thus the thickness of the less dense outermost portion of the Mo rear contact may be used to control the thickness of the resulting MoSe2 layer to the desired example range(s) discussed above.
According to certain further example embodiments, methods are provided for making a back contact for a photovoltaic device comprising: providing a rear substrate; depositing a back contact layer comprising a first Mo inclusive region having a first density and a second Mo inclusive region having a second density, said first density being greater than said second density, said second Mo inclusive region being formed at an outer portion of the back contact layer above the first Mo inclusive region, said second Mo inclusive region forming a foundational seed region, wherein a thickness of said foundational seed region comprising Mo having the second density has a thickness in a range selected from the group consisting of: from about 5-15 nm, from about 35-45 nm, from about 60-70 nm, and from about 90-100 nm. According to certain further example embodiments, this method may further include heat treatment in a selenium inclusive atmosphere to form a layer comprising or consisting essentially of MoSe2 out of the foundational seed layer, which also has a thickness in a range selected from the group consisting of: from about 5-15 nm, from about 35-45 nm, from about 60-70 nm, and from about 90-100 nm.
These and other embodiments and/or advantages are described herein with respect to certain example embodiments and with reference to the following drawings in which like reference numerals refer to like elements, and wherein: